Heat treated forging die having a low alloy content



July 7, 1970 A.. AF. FINKL ET AL. 3,519,499,"-A

HEAT TREATED FORGING DIE HAVING A LOW ALLOY CONTENT,

Filed April 19, 1966 United States Patent O 3,519,499 HEAT TREATED FORGING DIE HAVING A LOW ALLOY CONTENT Alfred F'. Finkl, Evanston, and William Wilson, Jr., Chicago, Ill., assignors to A. Finkl & Sons Co., Chicago,

Ill., a corporation of Illinois Filed Apr. 19, 1966, Ser. No. 543,661 Int. Cl. C04b 35/70; C22c 29/00 U.S. Cl. 148-36 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a low alloy steel especially adapted for hot work applications, and to a hot work implement made therefrom which is characterized in having a higher lower critical temperature than do implements made from comparative steels, and excellent uniformity of hardness throughout a cross section.

Although the concepts and principles of the present invention are applicable to many types and sizes of hot work implements the invention will be specifically described in terms of its application to closed die forging. It should be clearly understood, however, that the term hot work implement is intended to encompass any implement or tool which, during normal use, is subjected to an elevated temperature environment resulting from either intermittent or continuous contact with workpieces which are at temperatures substantially above room temperature.

There is a demand in the closed die forging industry for forging dies having better die life in order to increase production and reduce unit costs. In view of these demands the early failure of dies by heat checking, which is characterized by the appearance of a plurality of ne cracks in the die surface, and washing, which is the wearing away of the die so that the die impression goes oversize, has spurred the search for better implements. In hot forging operations for example the die surface may attain a working temperature of from about 600 to 1100 F. when operating at room temperature since it is exposed to intermittent contact with steel or other alloys which are at temperatures of around 2200 F. Other materials, such as titanium alloys, stainless steel, aluminum alloys, high temperature alloys, and Inconel may be forged at the same or lower temperatures. Some materials, such as the titanium alloys, additionally raise corrosion problems, and sometimes the dies are used to forge even higher temperature stock, such as tungsten alloys.

Forging die steel producers have long recognized that if the lower critical temperature of the die material can be elevated while all the other desirable operating characteristics are retained, beter die life can be expected. Die steel which, in operation, is heated to a point much above the critical temperature (as by overexposure with the workpiece such as will occur when the workpiece is locked in the dies) will reharden on cooling and may crack.

It is also well known in the art that the tempering temperature of the steel is related to the critical temperature in the sense that for a given hardness, the higher the lower critical temperature of the steel the higher the 3,5l9,499 Patented July 7, 1970 ice tempering temperature as a general rule. This factor is of importance in this industry because die steels intended for use in closed die forging are conventionally supplied to a specified hardness range, and therefore the higher thetempering temperature for a given hardness, the higher the temperature the die steel can withstand without'danger of sottening or cracking.

'It will be understood unless the context indicates otherwis'ethat the term lower critical temperature is intended to be interpreted in its usual sense, that is, as dening the' temperature at which a phase transformation from a ferritic or body centered cubic structure to an austinite or .face centered cubic structure occurs. As is well known, transformation of the steel will upset previous heat treatments.

Accordingly a primary object of the invention is to provide a low alloy die steel and a hot work implement made therefrom which has an increased resistance to softening or,in other words, an increased resistance to washing.

-Another object is to provide a low alloy die steel and a hot work implement made therefrom in which the danger of exceeding the lower critical temperature during work due to the presence of workpieces locked in dies or other factors, and the consequent loss of original heat treatment characteristics, and particularly hardness, is materially reduced.

Another object is to provide a hot work steel and a hot work implement made therefrom which is capable of forging relatively high temperature workpieces, such as tungsten alloys in the range of about 3,000 F.

A further object is to provide a low alloy steel and a hot work implement made therefrom as above described which has the ability to be hardened uniformly throughout a cross section thereof.

Yet a further object is to provide a steel and a hot work implement made thereof as above described having the above described advantages, a fine grain structure, and yet is low in cost.

Other objects and advantages will become apparent from the following detailed description of the invention.

The chemical composition of the steel with which this invention is concerned is, in its broadest form, substantially as follows:

C-from about .30 to about .45

Mn-from about .40 to about .80

Ni-from about .40 to about 1.00

Cr-from about 1.70 to about 2.60

Mo-from about .55 to about 1.20

V-from about .06 to about .20

Fe-balance, including P, S, and incidental impurities.

Carbon is necessary to provide the required hardness. If the eventual product is to be used in a range of 269 to 555 BHN, kwhich is the hardness range to which a comparable steel is used, it is necessary to employ a substantial quantity of carbon. If significantly less than .30` carbon is present it is difficult if not impossible to obtain the requisite combination of hardness and toughness. If more than .45 carbon is present the toughness of the steel may be significantly decreased. Preferably at least 33 carbon, and no more than .40 or .45 at the most, is employed. If the material is to be supplied in the hardness range of approximately 341 to 375 BHN, a carbon range of .33 to .38 is most desirable. Good toughness is necessary because the steel will fracture if the toughness is too low.

Manganese is desirable for combination with sulphur and should be present in the range of no less than .40 and no more than .80. If the manganese content is much below .45 surface problems may arise, such as cracking. On the other hand, if the manganese content is much above 3 .65 a refractory reaction may occur and adversely affect cleanliness, and there will be an adverse effect on the lower critical temperature. Accordingly it is preferred that manganese be present in the range of from about .40 to about .65, and more preferably in the range of from about .45 to about .65.

Silicon is an important element and should be present in rather wel] defined amounts. While silicon in the amounts of from about .20 to about .80 may be tolerated, it is preferable that silicon lie within the range of .30 t .60. If silicon is present in significant quantities above.,60 there is a tendency toward embrittlement. Most preferably silicon is present in amounts of from about .40 to about .60 and, if possible, it is preferred that the silicon content be near the upper end of the range.

Some nickel is needed to impart toughness to the steel. If significantly less than .50 Ni is employed the desired toughness may not be achieved. If much above 1.00 nickel is present the steel will be ake sensitive and tend to retain austinite. Furthermore, there may be an adverse effect on the lower critical temperature since increased amounts of nickel tend to reduce the lower critical temperature. A range of nickel from about .40 to 1.00 is contemplated although it is preferred that the nickel be present in the range `of from about .50 to about .80.

Chromium is an important element because it produces deep hardening and the lower critical temperature is raised by the presence of increasing quantities of chromium. High chromium contents however tend to lower the toughness of the steel. Accordingly, chromium is limited to a maximum of about 2.6% and a minimumof about 1.7%. In view of the rather substantial effect the presence of chromium has on the lower critical temperature and for purposes of deep hardening it is preferred that at least 2.00% be employed. For the best combination of depth of hardening, toughness, and lower critical temperature control, the optimum chromium content should preferably lie in the range of from about 2.10 to about 2.60.

Molybdenum tends to increase the depth of hardening. It also tends to resist softening in tempering operations and accordingly it tends to raise the lower critical temperature. If however the molybdenum content is much below .55, the effect on the critical temperature properties will be rather minimal. If more than 1.20 molybdenum is present the carbon content would have to be increased because molybdenum is a carbide former and a tendency to brittleness would develop. More preferably at least about .65 molybdenum is present. The upper content should more preferably be no greater than about 1.10, or most preferably, no greater than about .85.

Vanadium is desirable for its effect on grain size and its deoxidation ability. If the vanadium content is much less than .06 the desirable characteristics mentioned above will not be achieved. If the content is above about .20, additional carbon may be necessary because vanadium is a carbide former. Preferably the upper limit of vanadium should be no more than .16 since the undesirable characteristics of excess vanadium may appear through the upper end of, the aforementioned broad range. Experience indicates that vanadium is most desirably present in an amount of from about .06 to about .08 and, for the aforegoing reasons, and because it is a relatively expensive element, the last mentioned range is the most preferred range.

Two specific examples of the invention are as follows:

A heat of steel was made having the following compositions:

Fe-balance with the usual incidental impurities.

Two test bars of approximately 51/2" in length were forged from the above mentioned heat and placed in a furnace having a known temperature gradient therein. A similar test bar of a material melted into the following range was simultaneously subjected to the same furnace This standard steel is widely used in dies for closed die forging operations.

Referring to FIG. 1 it will be observed that the two test bars of this invention, indicated at 1 and 2, had an average lower critical temperature of 1365 F. whereas the test bar of the standard steel had a lower critical temperature of only 1340 F.

Die blocks were made from the same heat as the two test oars and used in closed die forging operations to forge the same parts as had been previously forged by die blocks made from the steel of test bar 3. No diiculty was encountered in die sinking and the material was sound and uniform throughout.

In another test, closed forging dies made from the new steel produced almost 6,000 track links before resinking was necessary. This is in contrast to normal production on the same part with the conventional steel of 4400 pieces per sinking.

In another test, a finisher die made from the new material produced 7,627 forgings as contrasted to a normal die life from the conventional steel of between 5,000 and 5,500 forgings. A tensile test cored from the finisher die gave the following results:

One of the most desirable features of the steel of the invention is its ability to harden throughout, that is, to have a substantially uniform hardness throughout cross sections. To illustrate, a central 17 x 18 slice was taken from a block measuring 131/2 x 17" x 18". Ten Brinnell hardness tests were taken from said block along a line parallel to the 18" dimension and substantially along the midline of the 17I side. The hardness readings are tabulated below:

Distance from BHN: reference edge, in. 302 .5 302 2.37 293 4.25 302 6.0 293 8.0 285 9.5 285 11.25 293 13.0 302 15,0 302 17.0

Another heat of the new steel was melted to the following composition:

HEAT B C-.3 6 Ni-.63 Mn-.54 Cr-2.55 P-.014 Mo-.75 S-.021 V-.07 4 Si-.52

Again two 51/2 bars, indicated at 4 and 5 in FIG. 2, were forged and placed in the above described furnace. It will be noted that bar 4 Ihad a lower critical temperature of 1425 F. and bar 5 had a lower critical temperature of l4l5 F., or an average lower critical temperature of l420 F.

It will thus be seen that the steel melted to the first experimental composition had a lower critical temperature of approximately 25 higher than the standard steel, whereas the steel melted to the second experimentalr composition had a lower critical temperature of 80 F. higher than the conventional steel.

While the heat treatment may be varied within limits it is preferred that it consist essentially of normalizing from l700 F., water or oil quenching from 1650 F. and double tempering to the desired hardness. As is apparent to those skilled in the art, the foregoing exemplary heat treatment involves heating to produce an austenitic structure, followed by the usual interrupted quench tinto at least the bainitic range, and subsequent tempering in the usual range of about 900 F.-1200 F. As will further be immediately apparent to those skilled in the artfrom the entire foregoing discussion, applicants invention` may be termed especially applicable to the field of massive forging dies for hot shaping.

Although va preferred embodiment of the invention has been illustrated and described it will at once be apparent to those skilled in the art that further modications may be made within the spirit and scope of the invention..

Accordingly it is intended that the invention be limited not by the scope of the foregoing description but solely by the scope of the hereinafter appended claims.

What is claimed is:

1. A massive forging die for hot shaping having substantially uniform hardness throughout, a lower critical temperature of no less than about 1365 F., and high resistance to wear and washing in the shaping of hot metals, and having a crystalline structure produced by heating to produce an austenitic structure followed by interrupted quenching into at least the bainitic range, and subsequently tempering in the usual range of about 900 F.- 1200 F., said alloy having the following approximate composition:

C-.30 to .45 Cr-1.70 to 2.60 Mn-.40 to .80 Mo-.55 to 1.20 Si-.ZO to .65 V-.06 to .20 Ni-.40 to 1.00

Fe--balance, including P, S, and incidental impurities.

2. A product as dened in claim 1 further characterfized, firstly, in having a lower critical temperature of no less than labout 1420 F. and, secondly, in that the individual components lie within the aforementioned ranges of claim 1 in the following relationship:

C-from about .33 to about .40

Mn-from about .40 to about .65

Si-from about .30 to about .60

Ni-from about .50 to about .80

Cr-from about 2.00 to about 2.60

Mo-from about .65 to about 1.10

V-from about .06 to about .16

Fe-balance, including P, S, and incidental impurities.

References Cited UNITED STATES PATENTS 1/1939 Ervin 75-128.85 7/1951 Gippert 75-128.85 3/1954 Brezin 75--128.85 1/1957l Hodge 75-128.85 8/1964 Bradd 75-128.85 4/ 1967 Manganello 75-128 HYLAND BIZOT, Primary Examiner U.S. Cl. X.R. 

